This has been covered many times in the forums and there's even a couple of wiki articles on the subject.

Read the Release Notes in detail to see which version you can upgrade from, incremental upgrades are never necessary unless stated in the Release Notes. Next, read the Certified wiki article on move a server from one server to another (plkus 32bit to 64bit, I'm assuming that's what you're doing as 32 bit versions of Zimbra are deprecated) and changing operating systems (there's only one article on this subject that's Zimbra certified, it's an NE document but also applies to the OSS version by omitting the NE steps.